Nocturnal core temperature during sleep is elevated during depression compared with remission in nonseasonally depressed patients. Similarly, nocturnal core temperature is higher during winter depression compared with remission induced by light treatment in seasonal affective disorder (SAD) patients. We investigated whether nocturnal core temperature in SAD patients naturally becomes lower in summer (during remission) compared with winter (during depression). Twenty-four-hour core temperature profiles were obtained in winter and summer in 22 SAD patients and 22 controls. The nocturnal core temperature minima were lower in summer compared with winter in SAD patients (p < .005), but not controls (p > .4). The seasonal changes in nocturnal core temperatures in SAD patients may reflect a unique physiological responsiveness of SAD patients to the change of seasons, and may be intimately related to the seasonal disturbances of mood and energy that are characteristic of SAD.